The present invention relates to a multi-stage compressor capacity control apparatus and more particularly to the multi-stage compressor capacity control apparatus suitably applied to the control of, for example, a multistage screw compressor that employs a starting unload method (starting load alleviating method).
The conventional unloader for the oil-free screw compressor, as mentioned in U.S. Pat. No. 3,367,562 and Japanese Patent Application Publication No. 93989/1984, utilizes the secondary pressure of the suction throttle valve, i.e., the negative pressure in a chamber 1c (see FIG. 3) downstream of the suction valve to cancel the start unload operation. This structure, however, has a long stroke of the suction throttle valve and lacks provision to ensure smooth transistion from the starting unload operation to the load operation (full-load operation) when the control pressure is very low.
Various problems accompanying the prior art are explained by referring to FIG. 3.
In FIG. 3, at starting when a sufficient control pressure is not obtained, transfer from the starting unload operation to the load operation is accomplished as follows. A starting unload reset command is issued to connect a chamber 1c with a chamber 1a through negative pressure connection pipe 18, three-way solenoid valves 9, 7 and operation pipe 16, putting the chambers 1a and 1c at the same negative pressure level, so that the suction valve 13 can easily be opened by a low pressure that develops in a chamber 1b.
The suction valve 13, as shown in FIG. 3, has two discs which offset the atmospheric pressure urging the suction valve 13 to close against the suction negative pressure so as to make the operating pressure of valve as small as possible (Japanese Patent Application Publication No. 249694/1985).
With this means alone, however, a problem is left unsolved that when the valve spindle 14b starts moving toward the right opening the suction valve 13, the pressure in the chamber 1c immediately reaches almost the atmospheric with the result that the suction valve 13 is not fully opened if the control pressure acting on the chamber 1a is at the level of atmospheric pressure.
There is also a possibility that when the compressor 2 is stopped during load operation with the outlet pressure in the compressor system at the atmospheric, i.e., with the service valve on the user side fully open, the pressure at the operation pressure extraction point 15 associated with the delivery pressure extraction port is not high enough to cause the suction valve 13 to fully close. This may lead to the compressor system failing to initiate the start unload operation when it is restarted next.